Enabling syntax and syntax folding in vim with large C file makes completion slow

Question

When I have syntax on in a large C file (about 8000) lines the completion ctrl-p and ctrl-n are very slow (more than 20). When I turn syntax off then completion takes less t

Answer 1

Vim is sometimes slow to (especially for ins-completion) in large files due to foldmethod=syntax

From :help todo:

• Slow combination of folding and PHP syntax highlighting. Script to reproduce it. Caused by "syntax sync fromstart" in combination with patch 7.2.274. (Christian Brabandt, 2010 May 27) Generally, folding with 'foldmethod' set to "syntax" is slow. Do profiling to find out why.

The FastFold plugin makes it so folds are only recalculated on save (so you're always using foldmethod=manual -- but the folds are calculated with foldmethod=syntax or whatever you had set before).

This solved the problem for me. Now I can use compl-whole-line completion in my 5000 line C++ file and it's instant and snappy instead of taking minutes and unresponsive.